1. Field of the Invention
The present invention relates to a micropattern forming method using an exposure apparatus for the manufacture of a semiconductor device and, more particularly, to a micropattern forming method whose precision is improved by multi-exposure method.
2. Description of the Related Art
In a reduction projection exposure apparatus which illuminates, with an illumination system, a mask on which an original image pattern is drawn, and transfers the pattern of the mask onto a wafer, a reduction in the size of the pattern to be transferred is required. In order to satisfy this requirement, decrease in wavelength of the exposure light and increase in NA (Numerical Aperture) have been performed.
A conventional exposure apparatus will be described with reference to FIG. 1. Light from a light source 71 is focused by a first focusing optical system 72 comprising an elliptic reflecting mirror, an input lens, and the like, and is uniformed by a uniforming optical system 73 comprising an optical integrator and the like. An aperture 75 that determines the shape of the light source is arranged on the exit of the uniforming optical system 73. Light passing through the aperture 75 is focused by a second focusing optical system 76 comprising an output lens, a collimator lens, and the like, and is radiated on a mask 77. Light transmitted through the mask 77 is projected on a wafer 79 through a projection optical system 78.
In this arrangement, when the light-incident side is observed from the position of the mask 77, the nature of the light incident on this position is the same as that of the light emerging from the uniforming optical system 73 through the second focusing optical system 76, and the exit of the uniforming optical system 73 appears to be a light source. Therefore, in the above arrangement, an exit 74 of the uniforming optical system 73 is generally called as a secondary light source.
When the pattern of the mask 77 is projected on the wafer 79, the projection exposure pattern forming characteristics, i.e., the resolution, the depth of focus are determined by an exposure wavelength .lambda., NA of the projection optical system 78, and the shape of the aperture 75 that determines the nature of the secondary light source 74. Generally, a resolution r and a depth of focus D of an exposure apparatus are given by the following equation: EQU r=A1..lambda./NA EQU D=A2..lambda./NA.sup.2
where A1 and A2 are parameters called process factors that are determined by the process conditions, the shape of the light source, and the like.
From these equations, it is apparent that in order to improve the resolution, the wavelength of the exposure light must be decreased and the NA must be increased. In this case, however, the depth of focus also decreases immediately.
It is known that in order to increase the depth of focus and improve resolution of a projection exposure apparatus, the phases of light transmitted through two adjacent transparent portions in a mask may be changed. Conventionally, a mask pattern that changes the phase of light transmitted through two adjacent transparent portions in a mask, called an alternating phase-shifting mask, is discussed in a literature entitled "Improving Resolution in Photolithography with a Phase-Shifting Mask" by Marc D. Levenson et al. in IEEE Trans on Electron Devices, Vol. ED-29 No. 12 (1982) p1828.
Typical examples of a mask structure and an exposing method for forming an optical image proposed in this literature will be described with reference to FIG. 2. A light-shielding film 82 is formed on a mask substrate 81, openings serving as the original image of the pattern are formed in the light-shielding film 82, and layers (to be referred to as shifters hereinafter) 83 for changing the phase of illumination light are formed on the opening regions. The shifters 83 are formed on every other one of adjacent openings regions. As the conditions of these shifters, a relation EQU d=.lambda./{2(n-1)}
must be satisfied where d is the film thickness, n is the refractive index, and .lambda. is the exposure wavelength. Light transmitted through openings around which the shifters are arranged and light transmitted through openings around which the shifters are not arranged have phases opposite from each other. Therefore, of the transmitted light, the light component corresponding to the pattern boundary has a light intensity of 0. In an image projected on a wafer 85 through a projection lens 84, the pattern has a clear contrast, thus improving the resolution.
When a negative resist as shown in FIG. 2 is used, a wiring pattern and the like can be formed. Note that a mask in which a desired resist pattern corresponds to an opening in this manner will be called a negative mask hereinafter. The above mask structure is a typical example and other various mask structures are also proposed. Even in other mask structures, image formation is performed in accordance with a similar optical principle. Reference numeral 86 in FIG. 2 denotes a resist pattern which is obtained by exposing a negative resist film to light through the above mask and developing the resultant structure.
In the periodic pattern represented by the above L/S pattern, shifters can be arranged around every other openings. However, in a pattern having an isolated opening, shifters cannot be arranged at optimum positions. Thus, as shown in FIGS. 3A to 3C, a method is proposed in which auxiliary openings 92 each having a line width less than the resolution limit are formed around an isolated opening 91, and a shifter is arranged on either the auxiliary openings 92 or the isolated opening 91, thereby improving the resolution (Jpn. Pat. Appln. KOKAI Publication No. 61-292643). After a resist film is exposed to a light pattern as shown in FIG. 3B through the mask shown in FIG. 3A, it is then developed as shown in FIG. 3C. A resist pattern 93 obtained in this manner has a very high resolution.
However, a method of this type has problems as follows. More specifically, the auxiliary patterns 92 must be formed as very thin patterns having a width less than the resolution limit so that they will not remain as resist patterns. Formation of such a mask imposes a heavy load. For example, when a device pattern having a minimum line width of 0.15 .mu.m is to be exposed through a X4 magnification mask under conditions of NA=0.5 and exposure wavelength=248 nm, although the width of the main opening of the mask is 0.6 .mu.m, the width of each auxiliary opening and the gap between the auxiliary opening and the main opening must be set to about 0.2 .mu.m on the mask.
In this manner, conventionally, in a phase-shifting method for resolving a thin isolated pattern, auxiliary openings having a width less than the resolution limit, i.e., thinner than the main opening, must be formed around the main opening of the mask, leading to a remarkable difficulty in fabricating the mask. Furthermore, a difference occurs in resolution characteristics between a periodic pattern, e.g., an L/S (line and space) pattern, and an isolated pattern, not only when the phase-shifting method described above is employed but also when the size of a pattern to be resolved becomes close to the resolution limit. In an actual LSI pattern, a periodic pattern, e.g., an L/S pattern, and an isolated pattern are mixed. Hence, if the exposure conditions are such that the L/S pattern is resolved with the correct mask size, the isolated pattern is not resolved with the correct mask size.